As integrated circuit features continue to shrink, test costs relentlessly rocket skyward. Greater numbers of interface nodes, higher operating frequencies, and specialized packaging arrangements such as multi-chip modules all contribute to soaring test costs.
To avoid wasting packaging materials and assembly costs, some tests are performed on an integrated circuit die prior to assembly. Equipment for testing integrated circuits before assembly increases in cost and complexity as die contacts decrease in size and increase in number. Indeed, the cost of exhaustive pre-assembly testing of every interface node of an integrated circuit is becoming prohibitive. Unfortunately, saving costs by reducing pre-assembly testing translates into increasing post-assembly waste.
Output buffers and input/output (I/O) buffers are features of an integrated circuit which typically must be thoroughly tested prior to sale. These buffers are ordinarily tested by test equipment which skews the relative relation between data signals and a clock or strobe signal until a failure is detected.
Source synchronous buffers are one type of buffer generally characterized in this manner. Source synchronous buffers operate by transmitting the strobe along with data from a driving chip to a receiving chip. With careful control over the signal paths, the transfer rate of information can be maximized because the strobe edge can be accurately placed to minimize the skew relative to the data.
At the destination, the strobe signal is used to create a window during which data should be captured. Manufacturing flaws or processing excursions adversely affecting the characteristics of the input or output circuitry may cause an input latch to fail to capture the data. For example, abnormal performance of a circuit which buffers the strobe signal could alter timing relationships sufficiently to cause failure under some operating conditions.
In order to detect such failures, prior art source synchronous testing typically uses expensive test equipment to provide data to each pin and appropriate strobe input. This scheme requires that a tester channel be assigned, or at least a tester interface be connected, to each buffer which is to be tested. This type of testing can be performed either before or after assembly. Prior art source synchronous buffer testing is also done in systems after the integrated circuits are assembled. This system testing also requires that each buffer tested is connected to another system component.
Thus prior art buffers can be characterized before packaging only by using expensive test equipment which contacts an interface node for each buffer. If testing is only performed after packaging, packaging materials and assembly costs may be wasted on parts having failures which could have been detected earlier.